1. Field of the Invention
This invention relates to a self-vibrating dc-operated brushless vibrator motor used for a pocketable wireless alerting or communication device such as a pager or a portable phone. The motor has a rotor having eccentric mass distribution that causes vibration.
2. Description of the Prior Art
A typical prior known dc vibrator motor has an eccentric weight made of a tungsten alloy fixedly mounted on an output shaft of the motor. The motor vibrates as the shaft rotates because of the unbalanced distribution of the mass of the rotor about the rotational axis.
However, a danger of mechanical interference pertains to this type of vibrator motor because of the existence of the externally rotating eccentric weight. In addition, the alerting device requires an extra space in which the eccentric weight rotates, and the high density material for the weight, such as tungsten alloy, is costly.
As disclosed in U.S. Pat. No. 5,036,239 (issued Jul. 30, 1991, one of the present applicants invented a flat coreless dc vibrator motor having neither output shaft nor external weight. This vibrator motor has a rotor which incorporates generally sector shaped non-overlapping flat armature coils arranged angularly partially about the shaft of the rotor, so that the center of mass of the rotor is off the rotational axis of the rotor.
As disclosed in U.S. Pat. No. 5,107,155 (issued Apr. 21, 1992), the same applicant of the present invention invented a dc vibrator motor without an output shaft. This vibrator motor has a fixed shaft on which a cylindrical magnet is fixedly mounted, and an eccentrically-weighted rotor having a cylindrical coreless winding, enclosing the fixed magnet, is mounted on the fixed shaft. The rotor has either eccentric bearings or a combination of eccentric bearings and eccentric weights, so that the center of mass of the rotor is off its rotational axis. Although the motor has been favorably accepted in the market, the rotor of this motor requires a substantial number of production process steps.
As disclosed in U.S. Pat. No. 5,341,057 (issued Aug. 23, 1994), the present applicants and another inventor jointly invented an additional dc vibrator motor without an output shaft. This vibrator motor has an eccentric armature rotor having three-pole armature iron cores and coils that are all situated within an angular range of 180.degree. about its rotational axis.
A sector-shaped eccentric armature rotor having all of the armature poles within an angular range of 180.degree. provides a good mass imbalance. However, the iron armature cores of the motor disclosed in U.S. Pat. No. 5,341,057 cause a cogging torque, a magnetic sticking effect, and, therefore, requires comparatively high voltage for starting.
Furthermore, all of the above vibrator motors disclosed in the U.S. patents have commutator brushes, thereby giving rise to a life span problem.
As disclosed in U.S. Pat. No. 5,373,207 (issued Dec. 13, 1994), the present applicants invented a dc brushless vibrator motor having neither an output shaft nor brushes. This vibrator motor employs a cylindrically-shaped coreless stator winding unit having at least three stator coils arranged at an even pitch angle and a rotor including an axially extended eccentrically-weighted permanent magnet having alternating north and south poles, angled partially, about the rotational axis. The rotor additionally includes a tubular yoke to provide magnetic flux paths.